White Spotting/Dominant White

→ Incomplete dominant means that a horse only needs one copy (W/n) of a W mutation to show a white pattern, but the amount of white is greater if a horse has two W mutations*.

General Information:

White Spotting (previously called Dominant White) is a general name given to a large number of white spotting patterns caused by mutations in the KIT gene.

27 “W” mutations are currently known. There is a large amount of variability in the amount/pattern of white caused by each of the mutations (W1-W27).

Sabino (SB1) is also caused by a KIT mutation, but due to naming conventions in early publications, it was given a different official name, and is not considered to be one of the “W” mutations. However, there are many “W” mutations that have “sabino-like” patterns.

Most “W” mutations are breed or line specific, having originated in one specific horse. Some of these horses did not produce offspring, so while we make all “W” tests available, it would be unlikely to show up in any other horse. We can advise you which tests will be appropriate for your horse.

*It is uncommon for a horse to have two “W” mutations, unless it is a specific “W” mutation in combination with the W20 mutation. It is believed that most combinations of “W” mutations are embryonic lethal, meaning the embryo does not develop and the mare simply cycles again as if she was not pregnant. This is different than the overo lethal white foal syndrome (OLWS) caused by the Frame Overo mutation.

There are two “W” mutations (W15, W19) which are recorded as being viable in homozygocity (having two copies). For example, one W15/W15 foal has been reported in the literature. CAG is studying “W” mutations and other causes of white spotting, and is very interested in this topic. If you have a foal that may be a combination of W mutations (eg. W15/W15, W15/W19, etc.) please contact us!

The exception to much of the above information is the W20 variant, which is found in many breeds. By itself it has very little influence on white patterning; instead, W20 acts as a “booster”: in combination with other “W” mutations, it can greatly increase the amount of white on a horse. It can also boost the amount of white in non-“W” patterns: (eg. Sabino-1, Tobiano, occasionally Frame Overo (when 2 copies are present:W20/W20)).

It is only possible to have two mutations in the KIT gene in one horse. This means that a horse can be, for example, W3/W20, or W20/W20 or SB1/W20, but it cannot be W3/W20/W20. There are two exceptions:

W22-W20: the W22 mutation occured on a chromosome which already had a W20 mutation. The W22 mutation is “stronger” than the W20 mutation (W22 results in the KIT gene being non-functional, instead of just less active), so W20 is generally not reported in this case. The horse would be W22/n, or W22/W20 if it had a W20 mutation on the other chromosome. It may be reported as W22/W20/W20, but this is rather meaningless/confusing, because the W22-W20 combination will always be inherited together.

Tobiano-W20: the Tobiano pattern is caused by a rearrangement of the chromosome (inversion) close to the KIT gene. Although very rare, it is possible to have a recombination event where Tobiano and W20 are then together on the same chromosome (Tobiano-W20). This would not be known unless the horse was tested for both Tobiano and W20, and two copies of W20 were detected. In this case, a report would indicate Tobinao-W20/W20.

Possible Genotypes:

Genotype:

Effect:

n/n

No White Spotting mutation

W/n

– One copy of any “W” variant → often “Sabino-like” pattern with a highly variable amount of white.
– One copy of the W20 variant → Usually does not cause white patterning on its own, but will increase the amount of white if the horse also has another white pattern.

W/W

– Two copies of any “W” variant → thought to cause early abortion of foal (except W15, W19, W20).
– Two copies of the W20 variant → sometimes allows a small amount of white (blaze, sock), but increases the amount of white if the horse has another pattern.
– One copy W20 with any other “W” variant (or other mutation on the KIT gene) → greatly increased amount of white.

Which White Spotting Mutation should I test my horse for?

→ The following list shows all of the “W” mutations reported in the current scientific literature. CAG offers tests for all of these mutations (W1 – W27). Each of the mutations (other than W20) is breed or line specific. If you have a horse which does not fit into the breeds or lines listed, or if you are uncertain of what you should test your horse for, we can advise you.

There are also “W” mutations that have not yet been identified. CAG is doing research into white spotting patterns, and if your horse has white spotting that is not caused by a known mutation, we will be very happy to add it to our research program. Contact us for more information!